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WASHINGTON -- Twelve people were killed and several others wounded Monday when a gunman identified as Aaron Alexis, 34, of Forth Worth, opened fire at the Navy Yard, authorities said.

In a news conference Monday afternoon, the chief medical officer at MedStar Washington Hospital Center said two of three shooting victims - including a DC Police officer - were in surgery. All three are expected to make a "good recovery."

During a news conference Monday night, officials said all the victims were civilians and were 46 to 73 years of age.

Washington Metropolitan Police Chief Cathy Lanier said one shooter was dead and one police officer was wounded in an "engagement" with at least one gunman at Naval Sea Systems Command headquarters. Federal officials identified the dead shooter as Alexis.

Lanier said authorities have "multiple" pieces of information indicating there could have been more shooters. She police were searching for a black man between 40 and 50 years old wearing an olive drab military-style uniform and carrying a long gun, Lanier said.

A federal law enforcement official said police killed Alexis, who was armed with an AR-15, a shotgun and a handgun. The federal official, who requested anonymity due to the fluid nature of the investigation, said there is no evidence that anyone other than the dead gunman fired weapons in the attack. The official said investigators were analyzing surveillance video of the shooting.

"What we lack is a sighting of somebody shooting, other than the dead shooter,'' said the official who is not authorized to comment publicly.

Alexis allegedly drove to the Navy Yard complex with the weapons early Monday and cleared security checkpoints before parking in a lot on the property, said the official, who was not authorized to comment publicly. After leaving his car, it is believed that Alexis was involved in two altercations in which he opened fire, killing one or possibly two people.

The official said Alexis then entered the building and went to the third and fourth floors, where much of the assault was carried out. He said Alexis did not appear to have an escape plan and it wasn't clear whether he was targeting specific people

Terry Durham said she was working in the Navy building when a fire alarm rang out. She was trying to leave when she encountered a shooter.

"We couldn't see his face, but we could see him with the rifle," Durham said. "He raised and aimed at us and fired. And he hit high on the wall."

Rick Mason, a program management analyst, said a gunman began shooting from a fourth-floor overlook in the hallway outside his office. He said the gunman was aiming down at people in the building's cafeteria on the first floor.

Patricia Ward, a logistics management specialist, said she was in the cafeteria. "I heard three shots -- pow, pow, pow. Thirty seconds later I heard four more shots."

Then panic, as people tried to get out of the cafeteria. "A lot of people were just panicking. There were no screams or anything because we were in shock."

Dave Sarr, an environmental engineer, was walking down a nearby street when he saw people running from the Navy Yard. Sarr has seen an evacuation drill a few days earlier at the Navy Yard. "At first I thought it was another drill," Sarr said. "Then I saw an officer with his weapon drawn."

President Obama made a brief statement, describing the victims as "patriots" and promising a thorough investigation. "I made it clear to my team that I want the investigation to be seamless," Obama said.

The first news broke with the Navy reporting on its Twitter feed that there was an "active shooter" at Building 197 at the Navy Yard, and that three shots had been fired at 8:20 a.m. ET. The Navy later reported deaths and injuries, but details remained fluid.

Flights at nearby Washington Reagan National Airport were disrupted, with all departures temporarily halted at the airport.

At MedStar Washington Hospital Center, chief medical officer Janis Orlowski said the hospital was treating three victims -- a male D.C. police officer and two women.

She said the police officer had multiple gunshot wounds to his legs and was in surgery. One woman was shot in the shoulder, and the other in the head and hand. All are expected to survive, she said.

The Navy Yard is located on the banks of the Anacostia River, a few blocks from the Nationals baseball stadium. It's in an urban area where the development of new parks, shops and apartments has been ongoing.

The Washington Nationals baseball team canceled Monday night's game against the Atlanta Braves. A parking lot at Nationals Stadium was being used as a site for families seeking to reunite with loved ones who work at the Navy Yard.

The city had not decided how long the area by the Navy Yard, including the baseball stadium, would remain closed to the public, said Keith St. Clair, communications director for the deputy mayor for public safety and justice.

Naval Sea Systems Command is the largest of the Navy's five system commands and accounts for a quarter of the Navy's entire budget. It builds, buys and maintains the Navy's ships and submarines and their combat systems.

NAVSEA headquarters' security requires guests to pass through turnstiles that are watched by security guards before entering. Visitors must also turn in their phones and other electronic recording devices upon entry.

Capt. Michael Graham, who works at NAVSEA, was running late this morning and by the time he arrived at work the base was already in a lockdown.

Graham said he had never seen a shelter-in-place drill in his five years at NAVSEA.

"I've never seen a shelter-in-place, I've seen the normal fire drills things like that, but never a shelter-in-place drill," said Graham. "Normally the drills you have are to get out of the building."

Marine Barracks Washington also put its base on a partial lockdown, only allowing Marines to leave if they were on official business, said Capt. Jack Norton, a base spokesman. A small contingent from Marine Barracks Washington's Guard Company serves at the Navy Yard, Norton said.

Defense Secretary Chuck Hagel issued the following statement:

I have been receiving regular updates on the shooting at the Washington Navy Yard, and continue to monitor the situation closely.

This is a tragic day for the Department of Defense, the national capital area, and the nation. My thoughts and prayers are with the victims of this outrageous act of violence, their families, and all those affected by today's events.

I am grateful for the swift response of federal and local law enforcement, and for the professionalism of DoD personnel at the Navy Yard complex.

The Department of Defense will continue to offer its full assistance in the investigation of this terrible and senseless violence.